LG John

Does having air-conditioning turned on affect performance? Obviously specifying it on a car will because it adds extra weight but does having it switched on at any one time have an effect (no matter how small). It uses more fuel but by that fact its not drawing any fuel that should otherwise be going into the engine so its fair to say that doesn't blunt performance. If I recall most/all air con units are run from a belt which obviously will sap some power but does switching it off disengage that belt??

This is just one of these random office conversations that needs a technical answer If i take the pug to Crail and put it on Eco mode will I get a 0.001 better time

pathetic shark

Yep - engaging the compressor via the clutch/drive belt will sap some performance, in exactly the same way (but with a different effect) as a supercharger/alternator etc

douglasb

Not a technical answer, but an answer from experience. Around 10 years ago, a colleague had a Pug 405 turbo diesel with aircon. Turning the aircon on had a definite effect. A few years later we both had 406 TDs. The aircon on them had a negligible effect on performance.

Buzzer

Aircon left on costs you about 1mpg

prana

definitely yes.

milo

the belt still runs regardless iirc... but its the clutch on the compressor drive pulley that makes the difference when switched on/off.

in my wrx the performance difference is VERY noticable especially when air con is first switched on

LG John

Eco mode it is then

Dracoro

IIRC, typically a car will lose around 5% of power. I'm guessing the figure will be more for crappier air con designs and and less for well engineered ones.

milo

yeah... the *** dyno would agree with that figure.

id love to see a proper dyno run with/without air-con switched on tho

ajm

Aircon systems can use up to 20-30 hp at max rpm and load - so yes they can affect performance!

Matt P

The belt runs all the time but load is only applied when the electromagnetic clutch is engaged (when you switch AC on) - that's the clunk that you can sometimes hear. I thought (and I may be wrong) that the scooby system disengaged the AC clutch at WOT so as not to draw power at critical times??

Tiggs

never turn the air off on mine.....why on earth would you?

JackClark

My WRX is bloody awful to drive with the aircon on. Bogs down when pulling off and won't stay at a stable cruising speed.

brickboy

On my old Volvo T4, there was an interlock so that the aircon would automatically switch OFF if you hit full throttle, and would re-engage a few seconds after you backed off.

I suppose the feature ensures you get max power available when you need it. I have read (can't remember where) that a modern aircon system on full whack takes about 10 -- 12bhp.

However, using aircon in the summer at cruising speeds saps less power and uses less fuel than the drag caused by opening windows / sunroof.

Mark Miwurdz

What if you took a pipe, connected it to the vents chucking out nice cold air, then fed it outside the car and back into the intake for the intercooler? Doubly cooled air...........

I'll get my coat

OllyK

Nicci

Have the aircon switched on while accelerating, then switch it off while still accelerating and you will feel the difference (and a free boost).

Thats how I noticed the difference it made (not actually a muppet post)

jaf01uk

I was in a Toyota Avensis diesel (mates company car) and he used the air con all the time in the summer but everytime it was mirror, signal, aircon off, overtake! I thought it was hilarious until he showed me the same manoeuvre with the air con on, scary!

LG John

I can't believe its that much of a difference!!! I switched mines off and it was just the same - not that I had it blasting out mental cold air in the first place though!

Neil Smalley

According to the manual,t he newer WRX's, STi's etc automatically switch off the air con when under heavy acceleration.

TopBanana

It probably wasn't engaged then, set it to max cold and try again. if you still can't fell the difference, that's a great reason not to worry about it!

ALi-B

iTrader: (1)

When working in commercial and industrial a/c like I have, you have to understand the kind of power an a/c compressor needs to run.

A typical small home/office unit (forget those crap mobile things) would pull about 3 to 5Kw of power, convert that in to Bhp and you roughly get bteween 4-7 Bhp power loss through the a/c on a car.

And to add to that on most systems you'll find the engine cooling fan or condensor fan will run almost continuously, sapping power via the alternator, say about 1 Bhp depending on battery condition and engine RPM. And also the interior (evaporator) fan too.

But obviously I'm using figures based an an ideal world system. A car based a/c system is likely to be far more inefficient than a electric home/office one. So even more power will be sapped. Latent heat, and that the compessor is not at run at a constant speed. Add to that we are'nt allowed to use R22 or R12 anymore and have to use R134a or whatever, which is not as efficient - thus requiring an even larger, more powerful system. So what was said as 4-7 Bhp can be anything like 8-14 Bhp (or even more) loss in the real world.

matt.bowey

Aircon will take away some of the engine power whilst the compressor is running.

You get better MPG using a/c than having a window open. Opening a window significantly increases drag, which the engine will have to overcome by using more fuel!

john banks

99/00 Scooby at least deactivates the aircon on full throttle.

Diesel

Bloody hell! Between 10 and 30 hp mentioned???????? That makes the 2Kw office aircon unit (for rather a larger area than inside a Scoob!”!!) look remarkably efficient? Hmmmmmmmmmmmmmmmmm? My fridge freezer too is only a few hundred watts......????????

Brit_in_Japan

A fridge freezer only has to cool a relatively small volume of air, and it's the same air it keeps extracting heat energy from so long as you don't open the doors. So it can do it little by little, the power required to do that is much less. A car air-con system has to pump a continual stream of cooled air and usually it's external air you are cooling, power requirement much higher.

Depending on your chosen comfort zone and external temperature your air-con system has to extract maybe 25 deg C from the air (have to cool the air to less than your chosen temperature because it mixes with existing warmer air in the car).

Also the sides of the fridge freezer are well insulated. Cars do not have double glazing or particularly good thermal insulation, so as a system it's quite inefficient.

FrenchBoy

if you want a quick test to see what the aircon does to your power, sit parked up with engine at idle - turn on your a/c and notice the engine increase its revs to compensate!

scoobyboy

there was a rumour a while back that a bloke in australia had connected his aircon to the inlet and it did make a difference in performance but weather it incresed his power by more than it took i dont know

douglasb

If he's increased the power by more than it's taken, the Sydney Patent Office can expect a visit from him with designs of a perpetual motion machine.

Doug

ALi-B

iTrader: (1)

What has always confused me with car a/c systems is: why don't they fit a reversing valve so it can heat as well...

why you may ask? well think about it, a freezing cold day, and a/c (heat pump) system will heat up far quicker than the coolant and will provide almost instant heat. Couple that to the cooling system and you can also use it to heat up the engine quicker on cold days. And thus raising cold start efficiency and lower cold start emissions quicker.

However, I suppose fitting a 2nd catalyst in the exhaust is an easier, cheaper and quicker solution to the problem in the eyes of manufacturers (even though it worsens efficiency)